When female sea lampreys look for suitable mates, a male's bile acid may be his most attractive feature, according to new research. During spawning season, these eel-like, parasitic fish migrate from open waters to streams, where males build themselves nests. Once they settle in, the males secrete a bilious love potion capable of attracting mates from afar. Although researchers have long suspected some kind of chemical communication between aquatic animals, this is the first evidence of water-released pheromones with long-range potency. These insights, detailed in the current issue of the journal Science, suggest novel possibilities for managing lamprey populations in regions such as the Great Lakes, where the parasitic fish have decimated local populations of salmon and trout.

Weiming Li of Michigan State University and his colleagues spent two years distilling a relatively tiny sample of the chemical secreted by male lampreys from more than a ton of water. They designed a simple experiment in which ovulated female lampreys placed in a watery maze could swim into one of two rooms. Each time a pheromone-releasing male swam upstream from one of the rooms, the female inevitably searched out the source of the bile, neglecting the regular water next door. The females exhibited nearly as much interest when the researchers treated the water in that same area with the purified pheromone compound, thus proving the viability of the sample. In contrast, the presence of a nonfertile male in one room had no effect on female choice.

Detailed chemical analysis of the compound also enabled Li's group to track the pheromone's probable source and pathway within the male. The bile acid most likely originates in the lamprey's liver. From there it travels through the bloodstream to the animals gills, which secrete the pheromone into the water, allowing it to flow downstream to expectant females. Li says that this new understanding of the lamprey mating process could be used to manipulate fish populations in an environmentally friendly manner.